In consideration of environmental influences, there has been a higher need of removing granular substances that are contained in an exhaust gas to be exhausted from an automobile engine or the like from the exhaust gas. In particular, the regulation regarding the removal of granular substances (particulate matters (PM)) to be discharged from a diesel engine tends to be more strict globally. Under such situations, a diesel particulate filter (DPF) for collecting and removing PM attracts attention.
As a mode of a DPF, there is proposed a filter (honeycomb filter) having a plugged honeycomb structure. This honeycomb filter is formed of a honeycomb structure in which there is provided a porous partition wall of defining and forming a plurality of cells acting as through channels of a fluid, having predetermined cells in which one end portion is open and the other end portion is plugged and remaining cells in which one end portion is plugged and the other end portion is open, the predetermined cells and remaining cells being alternately located. According to this type of honeycomb filter, a fluid (exhaust gas) flowing in the open one end portion of the predetermined cells, penetrating the partition wall, flowing out to the side of the remaining cells as a penetrated fluid, and then flowing out from the open other end portion of the remaining cells, thereby enabling the honeycomb filter to collect and remove PM in the fluid (exhaust gas).
In the above-mentioned honeycomb filter, the honeycomb filter has a structure in which an exhaust gas penetrates a porous partition wall (honeycomb filter of wall flow type) and, due to a large filtration area, the velocity of filtration (the velocity of penetration of the exhaust gas through the partition wall) is decreased and the pressure drop is small, as well as the collection efficiency of PM is comparatively good. However, these advantages are provided based on the assumption that in the porous partition wall of the honeycomb structure forming the honeycomb filter, there are no unintended pores larger than the pore diameter of the honeycomb structure (defects). In case of the presence of defects, PM passes therethrough before PM can be efficiently collected and the PM collection performance as a DPF is reduced. Therefore, an inspection for defects during the manufacturing of the honeycomb structure is a very important process. Incidentally, as a related prior art document, Patent Document 1 (Japanese Patent No. 3904933) can be proposed.